1. Field of the Invention
The present invention relates to a method of manufacturing layer-like structures in which a material layer having hollow cavities, preferably a porous material layer, is produced on a substrate consisting, for example, of monocrystalline p-type or n-type Si and in which the layer-like structure, or a part of it, is subsequently provided on the cavity exhibiting or porous material layer, and is subsequently separated from the substrate using the cavity exhibiting or porous layer as a position of intended separation, for example through the generation of a mechanical strain within or at a boundary surface of the cavity exhibiting or porous layer. Furthermore, the invention relates to different substrates which can be produced by this method and to novel semiconductor components which can be manufactured using the substrates of the invention.
2. Description of the Prior Art
A method of the initially named kind is known from several documents.
By way of example, a method of manufacturing a semiconductor body is described in the European patent application with the Publication No. 0 528 229 A1 in which a silicon substrate is made porous, a non-porous, mono-crystalline silicon layer is formed on the porous silicon substrate at a first temperature and in which a surface of the non-porous, monocrystalline silicon layer is bonded to a second substrate having an insulating material at its surface. Thereafter the porous silicon layer is removed by a chemical etching process and a further monocrystalline silicon layer is grown onto the first-named non-porous monocrystalline silicon layer by an epitaxial process at a second temperature.
The sense of this method is to be able to grow monocrystalline silicon on any desired substrate. The method is, however, relatively complicated because the porous silicon layer must be etched away. Similar methods are also apparent from the European patent specifications with the Publications Nos. 0 536 788 A1 and EP 0 449 589 A1.
In EP-A-0 767 486 a method of the initially named kind is described in which the porous layer has a region of increased porosity and the separation of the layer-like structure from the substrate takes place by mechanical separation in the region of increased porosity. The region of increased porosity is produced either by ion implantation or by a changed current density during the manufacture of the porous layer. Even if the method step of the separation can be improved hereby, the method is more complicated and an increased danger exists of undesired separation prior to or during the production of the layer-like structure. A multiple use of the starting substrate is admittedly achieved here; nevertheless one would, in many potential uses, use the expensive monocrystalline substrate in a relatively wasteful manner.
A similar proposal can be found from the non-prior published EP-A-0 797 258.
The manufacture of silicon solar cells at favorable cost requires high-quality silicon, as far as possible single crystal silicon for high photo voltages, thin Si-layers for material saving, but nevertheless adequate absorption, low manufacturing temperatures for energy saving and favorably priced foreign substrates, for example glass for the mechanical stability.
So far as is known, there are no methods which satisfy all these criteria. For example, work is described in some of the above-named European patent applications in which one carries out CVD epitaxy at temperatures above 800xc2x0 C. on porous silicon and transfers the so-formed epitaxial layers to a glass substrate. The silicon layers are not structured. For the separation, wet chemical processes or processes which destroy the substrate wafer are used. Applications in the photovoltaic field are not discussed.
The paper xe2x80x9cUltrathin crystalline silicon solar cells on glass substratesxe2x80x9d by Rolf Brendel, Ralf B. Bergmann, Peter Lxc3x6tgen, Michael Wolf and Jxc3xcrgen H. Werner, which appeared in Appl. Phys. Lett. 70(3), Jan. 20, 1997, describes a possibility of manufacturing structured polycrystalline silicon layers which are suitable for use as photocells. The paper is, however, not concerned with single crystal material and requires a complex structuring of the glass substrate and also a complex contacting of the p- and n-layers in order to realize the photocell.
Further documents which are concerned with porous silicon for different purposes include publications from the Research Centre Jxc3xclich, which deal with the manufacture of lateral diffraction gratings on the basis of porous silicon and interference filters of porous silicon. The paper xe2x80x9cOptical sensors based on porous silicon multi-layers: A prototypexe2x80x9d by W. Theixcex2, R. Arens-Fischer, S. Hilbrich, D. Scheyen, M. G. Berger, M. Krxc3xcger, M. Thxc3x6nissen, gives further information concerning the manufacture of porous silicon structures and possible applications of the so-produced structures. Thin layer silicon solar cells are moreover described in the publication xe2x80x9cCrystalline thin film silicon solar cells by ion-assisted depositionxe2x80x9d by S. Oelting, Dr. Martini and D. Bonnet. This publication appeared on the occasion of the xe2x80x9cTwelfth European Photovoltaic Solar Energy Conferencexe2x80x9d from Apr. 11-15, 1994.
The object of the present invention is to propose a method of the initially named kind which overcomes the above-named problems and enables the manufacture of components, in particular, but not only, silicon solar cells at favorable cost, with high quality silicon, so far as possible single crystal silicon, for photovoltages and thin silicon layers for material saving, but simultaneously achieving enhanced light absorption while using low manufacturing temperatures and cost favorable foreign substrates. In particular a method is aimed at in which the substrate used can be reused, or in which a plurality of like structures can be produced at favorable cost.
It is also an object of the present invention to propose methods for the production of different novel substrates which form the starting point for the production of further structures by means of epitaxial methods. Moreover, it is an object of the present invention to provide a photocell and other semiconductor components using the method of the invention which can be manufactured at favorable cost and which have excellent technical characteristics.
For the solution of this object methodwise, provision is made in accordance with the invention that the surface of the substrate is structured prior to the production of the porous layer or that the surface of the porous layer is structured.
Since the surface of the porous layer is structured, the mechanical separation at the boundary surface to the layer-like structure can apparently be carried out better, without need to produce porous layers with two different porosities. However, not only a mechanical separation comes into question, but rather also other methods which will be explained in more detail later.
Particularly important are the savings of time, effort and material which can be achieved by the use of structured layers, and indeed in particular when the structuring is exploited in the final product. Since the porous layer has a corresponding surface structuring, the layer-like structure can be provided with the same structuring.
In the manufacture of thin components with structured surfaces, only thin layer-like structures need then be produced. If one, however, operates in accordance with the prior art, which aims at planar surfaces, it is first necessary to produce thicker layers which must then be structured in a complicated manner by the removal of material.
That is to say, using the method of the invention, the porous layer can be made relatively thin, preferably in the range of about 100 nm to 10xcexc, so that relatively little material is lost and the working speed is improved, particularly since the once produced surface structuring of the substrate can be exploited for the manufacture of a plurality of identically structured layer-like structures.
When the separation of the layer-like structure from the substrate is carried out using mechanical stresses, then this separation takes place methodwise by means of the invention with structured surfaces in such a way that only the porous layer is damaged, but not the substrate or the layer-like structure. In many cases it is possible to carry out the separation at the upper boundary surface of the porous layer remote from the substrate so that the porous layer remains preserved. Accordingly, it is easy to reuse the substrate. For this, the porous layer is first removed, since it is as a rule damaged. A new porous layer is then produced on the substrate, after freeing it from the remainder of the porous layer, whereby the substrate can be reused. This is not possible in the prior art, when the porous layer is removed by etching or by mechanical removal from the layer-like structure.
It should be stated at this point that it would be possible to achieve this position of intended fracture or surface of intended fracture by a layer having hollow cavities instead by a porous layer, with it, for example, being possible to produce the hollow cavities by photolithography and for the hollow cavities to be open towards the free surface of the substrate. In this application only porous layers will now be discussed for the sake of simplicity. It should, however, be understood that these also include layers having hollow cavities and forming positions of intended fracture.
This type of separation of the layer-like structure from the substrate also succeeds when the surface of the porous layer is made flat. It is particularly favorable, in particular for the manufacture of the photocells or various other components, when the surface of the porous layer remote from the substrate is structured, since, on growing the layer-like structure on the porous layer, the layer-like structure reflects the structuring of the porous layer, so that, for example with a solar cell, the trapping of light takes place with substantially higher efficiency.
Since the structured surface of the substrate is preserved and can be reused, optionally after a cleaning step or after freshening up the structuring, a plurality of identical, layer-like structures can be manufactured from one substrate, which substantially increases the economy of the method, particularly since it is not necessary to newly structure the substrate every time.
The production of the structured surface of the porous layer can, in principle, take place in two ways. On the one hand, one can structure the surface of the single crystal substrate and then make it porous in manner known per se. The manufacturing process for the porous layer then automatically leads, with thin layers, to a porous layer having the same structuring as the structured substrate itself at its upper boundary surface remote from the substrate and at its lower (complementary) boundary surface facing the substrate. As an alternative to this, the planar surface of the single crystalline semiconductor substrate can be made porous, and the surface of the porous layer can be subsequently structured.
The substrate need not necessarily be monocrystalline, but can also be polycrystallinexe2x80x94assuming that the grain sizes of the polycrystalline material are larger than the width and thickness dimensions of the structuring, and the thickness of the porous layer, for example grain sizes of 100 xcexcm to centimeter size.
The typical structurings which come into consideration for solar cells have thickness and width differences which each lie in the range from 0.5xcexc to 100xcexc. Through the use of thin porous layers in the range from approximately 100 nm to 10xcexc, the shape of the porous surface of the porous layer remains true to the structured shape of the substrate, even with multiple use of the same, i.e. also with multiple production of the porous layers on one and the same substrate.
The layer-like structure is at least partly applied by an epitaxial method to the porous surface. The porous layer namely has the same crystalline structure as the original substrate and is well suited for the growth of layer-like structures by means of epitaxial methods, with the so grown layer-like structure then having the same crystalline structure, i.e. they are also monocrystalline.
The epitaxial method can be carried out as a homo-epitaxial method or as a hetero-epitaxial method. With hetero-epitaxy it is favorable that the porous layer can yield somewhat, so that a pronounced strain in the boundary surface region need not be feared.
Through the epitaxial method at least one semiconductor layer belonging to the layer-like structure is applied onto the surface of the porous layer. Depending on the purpose for which the layer-like structure is intended, other layers can then be applied onto the so-produced semiconductor layer, with it not being compulsory for these further layers to likewise have a monocrystalline structure. However, there are many structures in which the layer-like structure will consist of a plurality of monocrystalline semiconductor layers, for example two layers which form a p-n-junction.
It is, however, also possible, in accordance with the invention, to deposit a metal layer onto the layer-like structure and/or to apply a dielectric, for example in the form of a transparent or light-permeable window layer, for example through the Sol-Gel process or by means of an adhesive.
It is particularly favorable if a carrier layer is provided which is either brought into contact with the layer-like structure, for example by adhesive bonding, by wafer bonding or by a diffusion soldering process, or is formed as part of the layer-like structure, for example through a continuation of the epitaxial process. If the carrier layer is applied onto the surface of the layer-like structure by adhesive bonding, by wafer bonding or by a diffusion process, then it can, for example, consist of glass or aluminum. This carrier layer of the carrier will normally consist of a favorably priced and stable material, for example of glass. The mechanical separation of the layer-like structure from the substrate can then take place in that one, for example, pulls on the carrier layer or on the carrier, so that the carrier layer or the carrier with the layer-like structure separates from the substrate. The carrier layer or the carrier then forms a further substrate, on which the layer-like structure is provided. One can now carry out further method steps on the free surface of the layer-like structure. For example, if the layer-like structure represents a finished semiconductor element, this can simply be covered over or provided with a passivation or with surface contacts. This is of exceptional importance, because one can, by means of the invention, produce contacts, gates or electrodes on both sides of the layer-like structure, which brings many advantages both from a technical manufacturing point of view as well as with regard to the physical characteristics of the so-produced semiconductor components.
In the event that the layer-like structure is not yet finished, one can produce further semiconductor layers by epitaxial methods on the free surface of the layer-like structure and can optionally also effect further structuring by photolithographic methods or other methods, insofar as this is necessary. The crystalline structure of the layer-like structure is then retained during the further course of the epitaxial method.
As initially mentioned, the substrate with the remainder of the porous layer can then be used anew, after the separation of the layer-like structure from the substrate at the point of intended fracture that is provided, as a substrate for the application of a further layer-like structure.
The method can be particularly favorably further developed in that a further porous layer is produced on the surface of the layer-like structure remote from the substrate prior to or after the separation of the layer-like structure from the substrate, and a further layer-like structure can be provided on the further porous layer, with the method optionally being repeatable several times, whereby a plurality of layer-like structures, in particular structured layer-like structures, arise above one another, which are respectively separated from the adjacent layer-like structure by a porous layer forming a position of intended fracture, wherein, after the production of such a multiple structure, the individual layer-like structures can be separated from one another by the production of a mechanical stress within or at a boundary surface of the respective porous layer.
Through the production of the so-described multiple structure, a very rational manufacture of individual layer-like structures is achieved, which can then be separated one after the other from the multiple structure. That is to say, prior to the separation of the individual layer-like structures from the multiple structure, they are each provided with a carrier layer or are secured to a carrier, precisely as is the case when a single layer-like structure is formed on the substrate, as described in more detail above.
In this variant of the method, further structures can also be optionally grown by epitaxial methods on the free surfaces of the so-formed layer-like structures.
An alternative variant of the method of the invention is characterized in that one generates or applies a porous material layer out of or onto a first substrate, the layer optionally having a structured free surface, for example having grooves arranged parallel to one another, in that one applies a second substrate onto the free optionally structured surface of the porous material layer and subsequently separates the second substrate from the first substrate using the porous layer as a position of intended fracture by the production of a mechanical strain such that a layer or sections of the porous material layer remains or remain adhered on the second substrate, whereby the second substrate can be used for epitaxial methods.
It is particularly favorable if, after the separation of the second substrate from the first substrate, the residue of the porous layer is removed from the first substrate, a new porous layer is produced on the substrate and the above process is repeated, with this process being optionally repeatable a plurality of times in order to produce a plurality of second substrates starting from a first substrate.
Since the sections of the porous material layer remain bonded to each second substrate, any desired layer-like structures can be grown onto these substrates by means of the epitaxial methods. Since the alignment of the crystal structure in each section of the porous material layer is the same, the structures grown on the second substrates by epitaxial methods likewise have a monocrystalline structure, so that one can, starting from an expensive substrate, produce a plurality of substrates for epitaxial methods in a favorably priced manner.
Various possibilities exist for applying the second substrate onto the first substrate. One possibility lies in using an adhesive; another possibility would be to deposit a metal layer onto the porous surface of the first substrate and to then connect this metallic layer to a carrier material in a different manner. A carrier material can also be connected to the porous layer of the first substrate by means of a diffusion brazing process. It is only important that after the removal of the second substrate, sections of the porous material of the first substrate are present in distributed manner on the surface of the second substrate.
Various possibilities exist for the production of mechanical stress within the porous layer, which leads to the separation of the layer-like structure or a part of it from the substrate.
The substrates produced by the method are intermediate products, which are valuable in themselves.
The method of the invention is in particular used for the manufacture of high quality solar cells. A further possible use is a radiation detector.